Sensor for determining the concentration of oxidizable elements in a gas compound

ABSTRACT

Sensor for measuring the concentration of oxidizable constituents in a gas mixture, in particular for measuring one or more NO x , CO, H 2  gases, and preferably unsaturated hydrocarbons, by measuring the voltage between a measuring electrode a reference electrode or by measuring the voltage between two measuring electrodes. A porous solid electrolyte makes it possible to dispense with a reference gas atmosphere, thus providing greater miniaturization and simplifying the design. The selectivity toward individual measuring gas constituents can be improved by selecting the measuring electrode materials, in particular by using semiconductors.

FIELD OF THE INVENTION

The present invention relates to a sensor for measuring theconcentration of oxidizable constituents in a gas mixture, in particularfor measuring one or more of the gases NO_(x), CO, H₂, and preferablyunsaturated hydrocarbons.

BACKGROUND INFORMATION

Elevated concentrations of oxidizable constituents, in particular, ofNO_(x), CO, H₂, and hydrocarbons, can occur in the exhaust gases ofspark-ignition and diesel engines, internal-combustion machines andincineration plants, e.g. as the result of a component malfunction suchas an injection valve or as the result of incomplete combustion. Tooptimize the combustion reaction, it is therefore necessary to determinethe concentration of these exhaust gas constituents. A method formeasuring oxidizable gases is described in Unexamined Japanese PatentApplication No. 60-61654, in which a stoichiometric reaction with oxygentakes place at a first measuring electrode made of platinum-classmetals, and quasi-equilibrium states are established at one or moreadditional metallic measuring electrodes with reduced catalytic activityfor the oxygen equilibrium reaction. Nernst voltages E1 and E2 aremeasured between the measuring electrodes and a reference electrode,which is exposed to a reference gas having a constant oxygen partialpressure, and the concentrations of the gas constituents calculated fromthe difference between these voltages on the basis of calibrationcurves.

SUMMARY OF THE INVENTION

A sensor according to the present invention provides greaterminiaturization, a simplified design, and more cost-effective productionthan a conventional sensor. This is achieved according to the presentinvention by the fact that the solid electrolyte is porously sintered.The molecules of the measuring gas can thus diffuse through thesolid-electrolyte pores and reach the reference electrode, wherethermodynamic equilibrium is established. This exposes the referenceelectrode to an oxygen partial pressure which corresponds to thethermodynamic equilibrium. It is therefore not necessary to supply areference gas, which greatly simplifies the sensor layout.

The thermodynamic equilibrium can also be advantageously establishedeven in the solid electrolyte by selecting a catalytically active solidelectrolyte material. A particular advantage of this is that it enablesthe gases interfering with the reference signal to be selectivelyoxidized, simplifying signal analysis or even making it possible in thefirst place. Not only the solid electrolytes but also the measuringelectrodes can be suitably porous, which further improves the diffusionof the measuring gas molecules to the reference electrode.

Mixing additives made of the same substances as the electrodes into atleast one layer of the solid electrolyte which lie adjacent to theelectrodes improves electrode adhesion and thus also the service life ofthe sensor. It is also especially advantageous to construct themeasuring electrodes from semiconductors, which can significantlyincrease selectivity toward the individual gas constituents to bedetected.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a cross-section of a sensor according to the presentinvention.

DETAILED DESCRIPTION

FIG. 1 shows a cross-section of a sensor according to the presentinvention. One large surface of an insulating flat ceramic substrate 6contains, in vertically arranged layers, a reference electrode 5,preferably made of platinum, a porous solid electrolyte 4, measuringelectrodes 1 and 2, and a gas-permeable protective layer 3. A heatingdevice 7 with a cover 8 is mounted in the opposite large surface.

To measure the concentration of oxidizable constituents in exhaustgases, the sensor is heated to a temperature between 300 and 1,000° C.,advantageously to 600° C., using heating device 7.

The measuring gas diffuses through the porous solid electrolyte toreference electrode 5, which catalyzes the establishment of the oxygenequilibrium potential. Measuring electrodes 1, 2 are designed to havereduced catalytic activity in the oxygen equilibrium reaction. On atleast one measuring electrode, the sensor generates a cell voltage viathe oxygen ion conducting solid electrolyte by a first half-cellreaction initiated with the help of the reference electrode and a secondhalf-cell reaction influenced by the oxidizable gas constituents to bemeasured. The concentrations of the gas constituents is determined fromthe voltage values on the basis of the calibration curves.

In its simplest form, the sensor according to the present invention canbe used with one reference electrode which catalyzes the establishmentof gas mixture equilibrium and with one measuring electrode which cannotor can only slightly catalyze the establishment of gas mixtureequilibrium. However, it is also possible to install two measuringelectrodes, as shown in FIG. 1, or even multiple electrodes, each withdifferent catalytic activity for establishing oxygen equilibrium states.The measuring electrodes then respond with different voltages inrelation to the reference electrode, depending on the type of gas.

In arrangements with two or more measuring electrodes with differentcatalytic activities, it is also possible to evaluate voltages betweenthe measuring electrodes in order to measure oxidizable gases. Measuringvoltages between electrodes that are on the same plane and positionedequidistant from the heating device, such as electrodes 1 and 2 in FIG.1, also disables the Seebeck effect.

In arrangements with at least two measuring electrodes, it is alsopossible to completely or at least partially compensate forcross-sensitivity of a first measuring electrode us using the signal ofan additional measuring electrode by adjusting the sensitivity of thisadditional measuring electrode to the interfering gas constituents.

According to an additional embodiment, the solid electrolyte is designedin such a way, e.g. by mixing in 0.01% to 10% by volume of platinumpowder into at least one layer of the solid electrolyte facing thereference electrode, that the solid electrolyte catalytically convertsthe gases to be measured so that only the gases corresponding to thethermodynamic equilibrium reach the reference electrode, or that thesolid electrolyte converts only the gases interfering with the referencesignal.

According to an additional embodiment, one or more measuring electrodes,in addition to the solid electrolyte, are porous, thereby facilitatingthe diffusion of gas to the reference electrode.

Concerning the measuring electrode composition, it is possible to selectmetal electrode substances like those described, for example, inUnexamined Japanese Patent Application No. 60-61654, or semiconductorswith a high specific sensitivity toward certain oxidizable gases.Especially suitable are semiconducting oxides or mixed oxides, which canbe doped with an acceptor and/or donor having a concentration, forexample, of between 0.01% and 25%. The acceptor is incorporated into thesemiconductor as, for example, a solid solution or a segregatedconstituent. The high sensitivity, e.g. of acceptor- and donor-dopedn-type titanium oxide, in particular to unsaturated hydrocarbons, is dueto the adsorbent interaction between the orbitals of the pi bonds of theunsaturated hydrocarbons and the acceptor sites on the semiconductorsurface.

To prevent the conductivity-reducing acceptor component from becomingfully electronically active, it is advantageous to add to the electrodea conductivity-increasing donor, in particular in a higher concentrationthan the acceptor.

The following example describes a method for producing a sensoraccording to the present invention: rutile doped with 0.5% to 15%, forexample 7%, niobium and 0.25% to 7%, for example 3%, of one oftransition metals nickel, copper or iron is screen-printed in a 30 μmlayer onto a substrate on which are mounted a reference electrode,preferably made of platinum, and, on top if this, a solid electrolytelayer. A heating device is attached to the opposite side of thesubstrate. The sensor is sintered for 90 minutes at 1,200° C. with aheating/cooling ramp of 300° C./hour. After sintering, the solidelectrolyte has pores ranging in size from 10 nm to 100 μm.

An attached platinum conductor path, which is insulated from the solidelectrolyte and contacts only the measuring electrode, is used tomeasure the voltage between the reference and rutile electrodes at thecell formed in this manner with a resistance of 1 MOhm. In doing this,the sensor is heated by its heating device to a temperature of 600° C. Asimulated exhaust gas containing 10% oxygen, 5% water, and 5% carbondioxide, as well as 30 ppm sulfur dioxide, is used as the measuring gas.Oxidizable gases are then added in the quantities shown in the table.

For comparison, the last line in the following table shows the voltagevalues for a mixed-potential electrode made of 20% gold and 80%platinum, representing a measuring electrode according to the relatedart.

TABLE Voltage values (in mV) as a function of the concentration ofoxidizable gases and the composition of the measuring electrode Rutileelectrode with 7% Nb and 3% Reference oxidizable gases Voltages in mVelectrode, 20% (ppm) Ni Cu Fe Au and 80% Pt Propene 460 150 45 60 320180 120 36 47 280  90  90 27 35 180 H2 460  30 12 20 500 180  17  6 10450  90  5  3  4 380 CO 460  40  3 16  70 180  15 —  7  35  90  7 —  6 23

The table shows that a rutile semiconductor electrode with 7% niobium asthe donor and 3% nickel as the acceptor demonstrates the greatestsensitivity to propene as the conductive substance. The gold-platinumsystem known from the related art, on the other hand, shows especiallyhigh cross-sensitivity to hydrogen.

What is claimed is:
 1. A sensor for measuring a concentration of atleast one oxidizable constituent in a gas mixture, comprising: areference electrode catalyzing an establishment of a thermal equilibriumof the gas mixture; an ion-conducting solid electrolyte having at leastone pore, the solid electrolyte including additives which catalyzeoxidation of one of: the at least one oxidizable constituent, and the atleast one oxidizable constituent that interferes with a reference gas inthe gas mixture; at least one measuring electrode exposed to the atleast one oxidizable constituent, the at least one measuring electrodebeing substantially unable to catalyze the establishment of the thermalequilibrium of the gas mixture; and a flat electrically insulatingsubstrate, wherein the reference electrode, the solid electrolyte andthe at least one measuring electrode are situated in vertical layers ona large surface of the substrate.
 2. The sensor according to claim 1,wherein: the at least one oxidizable constituent includes at least oneof NO_(x), CO, H₂ gas and unsaturated hydrocarbons.
 3. The sensoraccording to claim 1, wherein a size of the at least one pore is between10 nm and 100 μm.
 4. The sensor according to claim 1, wherein: the atleast one measuring electrode has at least one pore.
 5. The sensoraccording to claim 4, further comprising: at least two measuringelectrodes.
 6. The sensor according to claim 4, wherein: the referenceelectrode is composed of a first material; the at least one measuringelectrode is composed of a second material; and the solid electrolyteincludes at least two vertically arranged layers, a first layer of theat least two layers facing the reference electrode and composed of athird material which includes the first material, a second layer of theat least two layers facing the at least one measuring electrode andcomposed of a fourth material which includes the second material, thefirst layer being different from the second layer.
 7. The sensoraccording to claim 1, wherein the at least one measuring electrodeincludes at least one semiconductor.
 8. The sensor according to claim 7,wherein the at least one semiconductor is doped with at least one of anacceptor and a donor.
 9. The sensor according to claim 8, wherein afirst concentration of the donor is higher than a second concentrationof the acceptor.
 10. The sensor according to claim 8, wherein the donoris composed of a first element having a first valence, the at least onesemiconductor being composed of a second element having a secondvalence, the first valence being higher than the second valence.
 11. Thesensor according to claim 10, wherein the donor is composed of at leastone of tantalum and niobium.
 12. The sensor according to claim 8,wherein: the at least one semiconductor is doped with the acceptor, theacceptor including at least one transition element.
 13. The sensoraccording to claim 12, wherein the at least one transition element is atleast one of nickel, copper, cobalt and chromium.
 14. The sensoraccording to claim 13, wherein the at least one transition element is atleast one of nickel, copper, cobalt and rare earths.
 15. The sensoraccording to claim 12, wherein: the acceptor is incorporated into the atleast one semiconductor as one of a solid solution and a segregatedconstituent.
 16. The sensor according to claim 8, wherein aconcentration of at least one of the acceptor and the donor is between0.01% and 25%.
 17. The sensor according claim 16, wherein the at leastone semiconductor is composed of 0.5% to 15% niobium and 0.25% to 7%nickel.
 18. The sensor according claim 16, wherein the at least onesemiconductor is composed of 7% niobium and 3% nickel.
 19. The sensoraccording claim 7, wherein the at least one semiconductor is composed ofone of an oxide, a single-phase mixed oxide and multi-phase mixed oxide.20. The sensor according claim 19, wherein the at least onesemiconductor is composed of one of a rutile oxide, a dirutile oxide anda further oxide, the further oxide including a mixture of the rutileoxide and the dirutile oxide.
 21. The sensor according to claim 19,wherein the at least one semiconductor is composed of a titanium oxide.